Hi Slapster
Assessors are quite accustomed to claimants' conditions changing from what they said on their form. They won't think you are trying to pull a fast one.
It is not so much about your condition, it is about how the symptoms affect your everyday life and mobility. Have you had a look at the Guide to PIP claims and Reviews?
benefitsandwork.co.uk/guides-for-claimants/pip If you start reading at page 109, you will find all the information about telephone assessments, what you need to do and be prepared for, and the types of questions you will be asked. I hope that you have asked for the assessment to be recorded. Even if you have, we advise that people record the telephone call themselves if they have the means, as sometimes the recordings go missing.
I hope it goes well for you.
BIS